Incandescent 3-Way light bulbs have been a popular product in the marketplace for many years because they offer the user the choice of three levels of light from one light bulb.
The incandescent 3-way light bulb consists of two filaments, a major and a minor. When used in a 3-way lamp socket, the contact point in the socket moves its position to make contact with the base of the light bulb at different locations. This change in locations will electrify each filament in turn, then both together. For example: A 3-way bulb rated at 30/70/100 watts utilizes a 70 watt major filament and a 30 watt minor filament. By switching the socket, first the 30 watt filament is electrified, then the 70 watt, then both together.
Typically this common design for 3 way light bulbs has proven to be a somewhat sensitive construction. This design is known for its early failure of at least one filament due to the constant switching of the filaments on and off and the limited supports to secure the filaments.